How the Tree Frog Has Redefined Our View of Biology

How the Tree Frog Has Redefined Our View of Biology

Karen Warkentin, wearing tall olive-green rubber boots, stands on the bank of a concrete-lined pond at the edge of the Panamanian rainforest. She pulls on a broad green leaf still attached to a branch and points out a shiny clutch of jellylike eggs. “These guys are hatchable,” she says.

Red-eyed tree frogs, Agalychnis callidryas, lay their eggs on foliage at the edge of ponds; when the tadpoles hatch, they fall into the water. Normally, an egg hatches six to seven days after it is laid. The ones that Warkentin is pointing to, judging from their size and shape, are about five days old, she says. Tiny bodies show through the clear gel-filled membrane. Under a microscope, the red hearts would just be visible.

She reaches down to wet her hand in the pond water. “They don’t really want to hatch,” she says, “but they can.” She pulls the leaf out over the water and gently runs a finger over the eggs.

Sproing! A tiny tadpole breaks out. It lands partway down the leaf, twitches and falls into the water. Another and another of its siblings follow. “It’s not something I get tired of watching,” Warkentin says.

With just a flick of her finger, Warkentin has demonstrated a phenomenon that is transforming biology. After decades of thinking of genes as a “blueprint”—the coded DNA strands dictate to our cells exactly what to do and when to do it—biologists are coming to terms with a confounding reality. Life, even an entity as seemingly simple as a frog egg, is flexible. It has options. At five days or so, red-eyed tree frog eggs, developing right on schedule, can suddenly take a different path if they detect vibrations from an attacking snake: They hatch early and try their luck in the pond below.

The egg’s surprising responsiveness epitomizes a revolutionary concept in biology called phenotypic plasticity, which is the flexibility an organism shows in translating its genes into physical features and actions. The phenotype is pretty much everything about an organism other than its genes (which scientists call the genotype). The concept of phenotypic plasticity serves as an antidote to simplistic cause-and-effect thinking about genes; it tries to explain how a gene or set of genes can give rise to multiple outcomes, depending partly on what the organism encounters in its environment. The study of evolution has so long centered on genes themselves that, Warkentin says, scientists have assumed that “individuals are different because they’re genetically different. But a lot of the variation out there comes from environmental effects.”

When a houseplant makes paler leaves in the sun and a water flea grows spines to protect against hungry fish, they’re showing phenotypic plasticity. Depending on the environment—whether there are snakes, hurricanes or food shortages to deal with—organisms can bring out different phenotypes. Nature or nurture? Well, both.

The realization has big implications for how scientists think about evolution. Phenotypic plasticity offers a solution to the crucial puzzle of how organisms adapt to environmental challenges, intentionally or not. And there is no more astonishing example of inborn flexibility than these frog eggs—blind masses of goo genetically programmed to develop and hatch like clockwork. Or so it seemed.

Dear Eduardo. Not sure if I agree with you – the finding is, on first reading, quite astonishing! Dr.Warkentin states . . . . . “Phenotypic plasticity offers a solution to the crucial puzzle of how organisms adapt to environmental challenges, intentionally or not”. . . . After 3 attempts to slowly read thru Prof.Dawkin’s book (’82), “The Extended Phenotype”, I gave up, as it was too difficult for me to understand. But perhaps you are correct Eduardo, when you say, ” . . . intrinsical to evolved dev genetic pathways”. Do you mean this frog egg finding is similar to the wonderful, ‘co-evolution’ fig/wasp adaptation story in, ‘A Garden Inclosed’ – “Climb’g Mt Improbable”? (My all-time favourite RD chapter A quote from pg276, “The fig tree causes an over-exploited fig to drop to the ground where all the wasp eggs inside the fig perish.” A truly fantastic adaptation.

No; recipe! Fields just disqualified herself from being taken seriously.

a revolutionary concept in biology called phenotypic plasticity

Revolutionary?! This idea is old. Wikipedia cites examples at least as far back as 1997, but I’m sure it was postulated and/or observed before then. Does anyone here know?

The concept of phenotypic plasticity serves as an antidote to simplistic cause-and-effect thinking about genes

It doesn’t mean there’s no causation; it means there are if-then clauses. Doesn’t Fields know how modern computers work?

scientists have assumed that “individuals are different because they’re genetically different. But a lot of the variation out there comes from environmental effects.”

Great. Now I don’t believe anything that Warkentin says, either.

Nature or nurture? Well, both.

Not only are scientists aware of non-genetic factors, including epigenetic factors if you define genetics classically; they are also aware that not all causation can be split into either the “nature” (non-environmental) or “nurture” (environmental) category, because effects are due to the *ineraction* between these two broad types of causes. You know – like if/then clauses? Funnily enough, they’re made possible by the fact that organisms have very large numbers of genes, which modify one another’s effects. Phenotypic plasticity therefore doesn’t undermine the importance of genetics. What’s more, genetics is of primary importance in evolution, because you don’t inherit environments (life isn’t Lamarckian).

Phenotypic plasticity offers a solution to the crucial puzzle of how organisms adapt to environmental challenges

No; it just gives a name to how they do it. To solve the puzzle, you have to in turn explain the origin of plasticity… which is epi/genetic!

Biologists used to think this kind of flexibility got in the way of studying evolution

Does anyone know what Agrawal is talking about?

A seed that isn’t as picky about its temperature and light requirements might do better in a new place—and might not have to wait for an adaptive mutation to come along.

This does not seem particularly remarkable. Sheep worried by dogs will give birth prematurely, and the lambs will be aborted if very early or premature survivors if near term. I don’t see it as remarkable that other embryos / foetuses should be able to react to stress prematurely with success.

The idea of the environment having a role on development is no new discovery. More likely this is another attempt to over-emphasize the influence of the environment in detriment of the critical genetic component. An idea that some authors have engaged in starting several years back and without much merit in my view. The whole notion of phenotypic plasticity as a distinct ingredient apart from conventional genetics can be easily rebutted with the same argument that Professor Dawkins reasoned to address an eventual discovery of a case of Lamarckism in “The Extended Phenotype”: at the end it would be just another adaptive mechanism gained through natural selection.

Eduardo Sibils Professor Dawkins reasoned to address an eventual discovery of a case of Lamarckism in “The Extended Phenotype”: at the end it would be just another adaptive mechanism gained through natural selection.

These epigenetic effects do seem rather minor compared to examples such as the sex of reptiles being determined by the temperature of the nest!